1. Field of the Invention
The present invention relates to an automatic transmission control system for automotive vehicles.
2. Description of Related Art
Automatic transmissions for, in particular, automotive vehicles comprise multiple speed transmission gear mechanisms, such as planetary gears. Such a multiple speed transmission gear mechanism includes a plurality of friction coupling elements, such as brakes and clutches, which are selectively activated to provide torque transmission paths for various gears. One of multiple speed automatic transmissions of this type is known from, for instance, Japanese Patent Publication No. 63-18053. It is typical for the multiple speed automatic transmission to be equipped with a torque converter.
Friction coupling elements of the multiple speed automatic transmission of this type are operated by a hydraulic control system. Each friction coupling element is locked with locking force variable according to pressure supplied thereto. While the locking pressure must be higher than a specific level, it is undesirable for the friction coupling element to be supplied with an excessive level of pressure from the view point of fuel efficiency and shift shock. For this reason, the locking pressure is typically developed according to engine output torque.
The friction coupling element must be coupled with coupling or locking force appropriately strong to transmit the engine output torque to the transmission gear mechanism without causing slippage. For stable operation of the transmission gear mechanism, the hydraulic pressure is practically supplied to the frictional coupling element so as to generate the locking force slightly higher than the appropriate level. Because the locking force widely different from the appropriate level causes aggravation of fuel efficiency, the hydraulic pressure is altered according to the engine output torque so as to cope with a change in the appropriate level.
In a region where the engine output torque is small, as the engine throttle increases its opening, and hence the engine increases its output torque, the state of torque transmission shifts from an ordinary load region where torque is transmitted to the transmission from the engine to a reverse load region where torque is transmitted from the transmission to the engine. The locking pressure may be a zero level when the engine output torque reaches a transitional level at which the state of torque transmission is changed over between the ordinary and reverse load regions. In the ordinary load region where the engine produces output torque greater than the transitional level, the locking pressure is increased with an increase in the engine output torque.
If the locking pressure greater than the appropriate level is supplied to a friction coupling element during a gear shift, the automatic transmission produces what is called shift shock. For this reason, during a gear shift, the locking pressure is typically brought close to but above the appropriate level by reducing a margin of pressure for stable operation.
When stepping on an accelerator pedal with an intention to increase the engine output torque during a gear shift where the pressure margin is small, such as an up-shift, the friction coupling element experiences a lack of torque transmission capacity which always results in an increase in turbine speed. Accordingly, the automatic transmission loses the engine torque by an amount spent on the increase in turbine speed. Because the lose of input torque to the transmission is, however, ignored in the determination of the locking pressure based on the engine output torque, a change in locking pressure becomes greater than a change in the input torque, the friction coupling pressure is supplied with the locking pressure in excess. In particular, in an extent of small engine output torque which ranges both the in ordinary load region and reverse load region, when the accelerator pedal is stepped on with a result of changing over from the reverse load region to the ordinary load region during a gear shift, in particular an up-shift, an excess of locking pressure becomes more significant, causing sharp or sudden shift shock.